wlserver: check wlr_xwayland_server_create for failure

[adamsjo95]

wlr_xwayland_server_create() returns NULL on failure, but
gamescope_xwayland_server_t's constructor uses the return value
unchecked, so the next line dereferences NULL and gamescope segfaults
with no indication of the cause.

wlroots' own caller checks it (xwayland/xwayland.c), and the function
has four return NULL paths — missing Xwayland binary, allocation
failure, and two error labels.

Reproduction

Any condition that makes Xwayland server creation fail will trigger it.
I hit it with a /tmp/.X11-unix that wlroots rejects on ownership
grounds, which exhausts all 33 display slots:

[Error] wlserver: [xwayland/sockets.c:83] /tmp/.X11-unix not owned by root or us
... (x33)
[Error] wlserver: [xwayland/sockets.c:200] No display available in the first 33
Segmentation fault (core dumped)

Backtrace before the patch:

#0  wl_signal_add (signal=0x90, listener=0x...) at wayland-server-core.h:490
#1  gamescope_xwayland_server_t::gamescope_xwayland_server_t (...) at src/wlserver.cpp:1816
#2  std::make_unique<gamescope_xwayland_server_t, wl_display*&, int&> ()
#3  wlserver_init () at src/wlserver.cpp:2204
#4  main (...) at src/main.cpp:1062

signal=0x90 is the offset of events.ready on a NULL base.

After

[Error] wlserver: [xwayland/sockets.c:200] No display available in the first 33
[Error] wlserver: Failed to create Xwayland server
abort (core dumped)

Exit status goes from 139 (SIGSEGV) to 134 (SIGABRT), with the cause
named.

Notes

I used abort() to match the wlr_output_commit_state() failure
handling twelve lines below in the same constructor. Happy to change it
to a cleaner exit if preferred — this failure is often a user-fixable
environment problem rather than an internal invariant violation, so a
non-abort path may read better.

Tested on gamescope fcc1341, wlroots 0.20.2. Builds clean with
--werror. meson test --suite gamescope passes 2/2 before and after.